In this PhD thesis, the recombination of different atomic lattices in stacked 2D materials such as twisted bilayer graphene is studied. Using the different possibilities of Low-Energy Electron... Show moreIn this PhD thesis, the recombination of different atomic lattices in stacked 2D materials such as twisted bilayer graphene is studied. Using the different possibilities of Low-Energy Electron Microscopy (LEEM), the domain forming between the two atomic layers with small differences is studied. Superlattices in three such 2D material systems are studied. In twisted bilayer graphene, the small difference is caused by a twist of approximately one degree between the layers. In graphene on SiC, the difference is caused by the lattice mismatch between a buffer layer bound to the substrate and the next graphene layer. For both, we show that domains of different shapes and sizes occur and relate them to strain and lattice mismatch. The third system studied is tantalum disulfide. In this layered material, two different superlattices occur: a superlattice between atomic layers with different atomic arrangements in the layers, so-called polytypes, and the superlattices between the atomic lattice and the Charge Density Waves (CDW). CDWs cause a large temperature dependent resistivity change. The influence of a mixture of different polytypes on the precise CDW states is studied using LEEM spectroscopy and local Low-Energy Electron Diffraction. Show less
Jong, T.A. de; Chen, X.; Jobst, J.; Krasovskii, E.E.; Tromp, R.M.; Molen, S.J. van der 2022
Stacking domain boundaries occur in Van der Waals heterostacks whenever there is a twist angle or lattice mismatch between subsequent layers. Not only can these domain boundaries host topological... Show moreStacking domain boundaries occur in Van der Waals heterostacks whenever there is a twist angle or lattice mismatch between subsequent layers. Not only can these domain boundaries host topological edge states, imaging them has been instrumental to determine local variations in twisted bilayer graphene. Here, we analyse the mechanisms causing stacking domain boundary contrast in Bright Field Low-Energy Electron Microscopy (BF-LEEM) for both graphene on SiC, where domain boundaries are caused by strain and for twisted few layer graphene. We show that when domain boundaries are between the top two graphene layers, BF-LEEM contrast is observed due to amplitude contrast and corresponds well to calculations of the contrast based purely on the local stacking in the domain boundary. Conversely, for deeper-lying domain boundaries, amplitude contrast only provides a weak distinction between the inequivalent stackings in the domains themselves. However, for small domains phase contrast, where electrons from different parts of the unit cell interfere causes a very strong contrast. We derive a general rule-of-thumb of expected BF-LEEM contrast for domain boundaries in Van der Waals materials. Show less
